1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode, and more particularly to a nitride semiconductor light emitting diode that adopts a flip chip structure. Also, the present invention relates to a method of manufacturing the same.
2. Description of the Related Art
Generally, a nitride semiconductor light emitting diode is a light emitting diode used to obtain light having a blue or green wavelength band. The nitride semiconductor light emitting diode is made of a nitride semiconductor material having the following formula:AlxInyGa(1-x-y)N
Where, 0≦x≦1, 0≦y≦1, and 0≦x+y≦1
A substrate used to grow such a nitride semiconductor material is limited to a sapphire substrate because of lattice matching. The sapphire substrate is an insulated substrate, and thus both electrodes of the nitride semiconductor light emitting diode are formed at the opposite side of the sapphire substrate, i.e., a crystal growth surface. Considering the above-mentioned structural characteristics, there has been briskly developed a nitride semiconductor light emitting diode adopting a flip chip structure where the sapphire substrate is provided with a main light emitting surface.
FIG. 1 shows a flip chip structure of a conventional flip chip-type nitride light emitting diode.
A flip chip-type light emitting device 20 shown in FIG. 1 comprises a nitride semiconductor light emitting diode 10 mounted on a chip substrate 21. The nitride semiconductor light emitting diode 10 comprises a sapphire substrate 11, an n-type nitride semiconductor layer 12 disposed on the sapphire substrate 11, an active layer 13 disposed on the n-type nitride semiconductor layer 12, and a p-type nitride semiconductor layer 14 disposed on the active layer 13. The nitride semiconductor light emitting diode 10 is mounted on the chip substrate such that electrodes 19a and 19b are connected to lead patterns 22a and 22b via conductive bumps 24a and 24b, respectively. The sapphire substrate 11 of the light emitting diode 10 is a light transmittance substrate. Consequently, the sapphire substrate 11 may be used as a light emitting surface of the flip chip-type light emitting device 20.
It is required that the electrodes of the nitride semiconductor light emitting diode 10, especially, the p-side electrode form ohmic contact with the p-type nitride semiconductor layer 14, and have high reflexibility sufficient to reflect light emitted from the active layer 13 to the sapphire substrate 11.
As shown in FIG. 1, therefore, the p-side electrode structure may include an ohmic contact layer 16 and a metal barrier layer 17 formed on the p-type nitride semiconductor layer 14. The ohmic contact layer 16 is made of Ag or Al having high reflexibility of approximately 90% to 95% so that the ohmic contact layer 16 can improve contact resistance. The metal barrier layer 17 serves to prevent undesired immigration of components of the ohmic contact layer 16.
However, the metal, such as Ag or Al, forming the ohmic contact layer 16 is very sensitive to heat. As a result, the reflexibility of the ohmic contact layer 16 is lowered to approximately 60%-80%. If the reflexibility of the ohmic contact layer 16 is lowered, brightness of the flip chip-type light emitting device 20 is also lowered.
Also, the conventional nitride semiconductor light emitting diode 10 substantially has a planner electrode structure. Specifically, the p-side ohmic contact layer 16 has specific resistance lower than that of the p-type nitride semiconductor layer 14. As a result, a large portion of electric current is concentrated to a part A adjacent to the n-side electrode as indicated by the arrow. In other words, current crowding occurs. When the current crowding occurs, forward voltage is increased, light emitting efficiency of the active layer 13 disposed opposite to the n-side electrode 19a is decreased, and heat value of the part A where the electric current is concentrated is increased, whereby reliability of the nitride semiconductor light emitting diode is considerably deteriorated.